1. Technical Field
The present invention generally relates to light emitting devices and, particularly, to a solid-state light emitting device with high light extraction efficiency.
2. Discussion of Related Art
Referring to FIGS. 3 and 4, a typical solid-state light emitting device, e.g., a semiconductor light emitting diode (LED) 600 is illustrated. The LED 600 includes an dielectric substrate 610, a light emitting structure 620 formed on the dielectric substrate 610, an N-electrode 630 and a P-electrode 640. The light emitting structure 620 includes an N-type layer 621, a P-type layer 623 and an active layer 622 sandwiched therebetween. The N-type layer 621 has an exposed portion uncovered by the active layer 622 and the P-type layer 623. The N-electrode 630 is an N-contact pad and formed on the exposed portion of the N-type layer 621. The P-electrode 640 is a P-contact pad and arranged on the P-type layer 623. The N-electrode 630 and the P-electrode 640 are diagonally arranged with each other. However, due to a relatively high resistance of the P-type layer 623, the current injected at the P-electrode 640 does not spread laterally away from the P-electrode 640 and does not spread laterally within the P-type layer 623. Instead, the current flows from the P-electrode 640 along the shortest path (i.e., usually vertically) across the active layer 622 to the N-type layer 621. As a result, an area utilization ratio of the active layer 622 is lowered and thus the light extraction efficiency of the light emitting diode 600, especially with regard to a light emitting diode with a large sized light emitting structure, is degraded.
Referring to FIGS. 5 and 6, in order to overcome the above-mentioned issue associated with the low area utilization ratio of the active layer, an improved light emitting diode 700 is disclosed. The light emitting diode 700 includes a light emitting structure 720 having an N-type layer 721 and a P-type layer 723, a comb-shaped N-electrode 730 and a comb-shaped P-electrode 740. The comb-shaped N-electrode 730 and P-electrode 740 are interdigitated with each other and respectively ohmic contacted with the N-type layer 721 and the P-type layer 723. The use of the comb-shaped N-electrode 730 and P-electrode 740 can macroscopically improve current spreading to some degree, but still subject to current crowding in the microscopical level of the comb-shaped P-electrode 740. Accordingly, the light extraction efficiency of the light emitting diode 700 is low.
Therefore, what is needed is a solid-state light emitting device with good current spreading and thus relatively high light extraction efficiency.